1. Technical Field
The present invention relates to a signal-processing circuit of an analog type, in particular operating as a rectifier and peak detector, with active elements and differential inputs.
2. Description of the Related Art
Rectifier and peak-detector circuits are known which enable extraction from an analog signal (for example, a voltage signal) of information correlated to its mean amplitude or else to its peak value. In a known way, these circuits are generally based upon the use of unidirectional current-conduction means (for example, diodes) for rectifying the signal, and of charge-storage means (for example, capacitors) for detecting the peak value.
It is also common to use analog circuits of a fully-differential type, i.e., ones provided with pairs of differential inputs and outputs and in particular supplying at output a first differential signal VP(t) and a second differential signal VM(t), which have the same amplitude but are in phase opposition with respect to one another, and vary (in a way opposite with respect to one another) around a common-mode voltage VCM. The use of fully-differential structures is advantageous in so far as it enables doubling of the dynamic range of the output signals and an automatic rejection of disturbance common to the two differential inputs. For example, circuit blocks of a fully-differential type are typically associated to the outputs of accelerometric sensors, gyroscopes, or other inertial sensors, for performing preliminary processing operations of corresponding output signals.
In particular, in some applications, it is required to obtain, from the pair formed by the first differential signal VP(t) and the second differential signal VM(t), a second pair of signals, a first peak signal Vpp(t) and a second peak signal Vpm(t), which are respectively equal to the positive peak (with respect to the common-mode signal VCM) of the first differential signal VP(t) and to the negative peak (once again with respect to the common-mode signal VCM) of the second differential signal VM(t).
FIG. 1 shows a peak-detector circuit 1 of a known type, having a first input 2a, a second input 2b, and a third input 2c, which receive, from a generic time-continuous and fully-differential analog block 3, respectively the first differential signal VP(t), the second differential signal VM(t), and the common-mode signal VCM, which has a substantially constant value. The peak-detector circuit 1 has a first output 4a and a second output 4b on which it supplies, respectively, the first peak signal Vpp(t) and the second peak signal Vpm(t).
In detail, the peak-detector circuit 1 comprises a first detection circuit branch 1a and a second detection circuit branch 1b, which are identical to one another, are completely separate and independent, are connected to the first input 2a and to the second input 2b, respectively, and are both connected to the third input 2c. 
Each detection circuit branch 1a, 1b comprises: an operational amplifier 5a, 5b having its non-inverting input connected to the first/second input 2a, 2b and its inverting input connected in feedback mode to the first/second output 4a, 4b; a diode 6a, 6b connected between the output of the operational amplifier 5a, 5b and the first/second output 4a, 4b (in particular, the diode 6a has its anode connected to the output of the corresponding amplifier 5a and its cathode connected to the first output 4a, and the diode 6b has its cathode connected to the second output 4b and its anode connected to the output of the corresponding amplifier 5b); a capacitor 8a, 8b, connected between the third input 2c and the anode/cathode of the corresponding diode 6a, 6b; and a current generator 9a, 9b connected between the first/second output 4a, 4b and, respectively, a reference voltage GND and a supply voltage VDD.
Operation of the peak-detector circuit 1 (reference may be made also to FIG. 2 in which the waveforms of corresponding electrical signals are shown) is now described with specific reference to the first detection circuit branch 1a (but similar considerations may be applied to the second detection circuit branch 1b).
When the first differential signal VP(t) is higher than the first peak signal Vpp(t) present on the first output 4a, the diode 6a conducts, and the current at output from the amplifier 5a charges the capacitor 8a, causing rising of the first peak signal Vpp(t), which “follows” the first differential signal VP(t). As soon as the first differential signal VP(t) drops below the first peak signal Vpp(t), current conduction through the diode 6a is interrupted, and the capacitor 8a discharges through the current generator 9a, which extracts a current having a very low intensity (so that the first peak signal Vpp(t) will decrease very slowly). The flow of current that charges the capacitor 8a resumes as soon as the first differential signal VP(t) again exceeds the value of the first peak signal Vpp(t), which coincides with the value of the residual voltage on the capacitor 8a added to the common-mode voltage VCM.
Operation of the second detection circuit branch 1b is similar, with the difference that the flow of current at output from the amplifier 5b discharges the corresponding capacitor 8b when the second differential signal VM(t) is lower than the second peak signal Vpm(t) present on the second output 4b, and the current supplied by the current generator 9b charges the capacitor 8b. 
It is evident that in both of the circuit branches the first and second current generators 9a, 9b can be constant-current sources or else can be replaced with resistors.
The circuit described has a number of drawbacks. In the first place, a problem is represented by the high area occupation, due to the need to replicate twice one and the same peak-detector circuit. In fact, the two differential signals at input are used separately for detecting the positive peak signal and the negative peak signal. In addition, the two detection circuit branches 1a, 1b are separate and independent, and there consequently exists the risk of “mismatch” between component values, which can cause offsets between the first and second peak signals Vpp(t), Vpm(t). In addition, each peak-detection circuit branch 1a, 1b functions as a half-wave rectifier, operating on just one peak (positive or negative) of the input voltage, respectively when VP(t)>VCM, or else when VM(t)<VCM. As compared to a full-wave rectifier, the amplitude of the output-voltage ripple (designated by r in FIG. 2) is evidently greater as the time of discharging/charging of the capacitor 8a, 8b increases.